The present invention relates to body insertable catheters and other tubular treatment devices, and more particularly to the manner in which such devices can be reinforced with metallic wires or strands.
Procedures involving entry into or treatment of body lumens, e.g., intravascular treatment procedures, frequently employ catheters. A variety of catheter types have been developed to meet different treatment needs. For example, U.S. Pat. No. 4,547,193 discloses an angiographic catheter. U.S. Pat. No. 5,364,357 discloses a small diameter coronary angioplasty catheter, more particularly a balloon dilatation catheter for expanding vascular passages. U.S. Pat. No. 5,221,270 is drawn to a soft tip guiding catheter, which, after its insertion into a body lumen, is used to receive other devices and guide them through the same lumen. U.S. Pat. No. 5,026,377 discloses a catheter used to deploy and position a self-expanding stent in a body lumen.
The variety of uses notwithstanding, certain properties are necessary or desirable, regardless of the catheter type. All catheters must be biocompatible, with vascular catheters further requiring hemocompatibility. To varying degrees, catheters require sufficient axial strength to facilitate movement of the catheter distal end through vascular passages or other body lumens by applying an axial pushing force near the catheter proximal end. This characteristic is often called xe2x80x9cpushability.xe2x80x9d A related characteristic, xe2x80x9ctorqueability,xe2x80x9d refers to the ability to rotate the catheter distal end by rotating its proximal end. Torqueability is useful, for example, in using a helical fixation member to temporarily or permanently secure a catheter to tissue. Flexibility, particularly along a distal end region of the catheter, becomes increasingly important as the catheter enters more serpentine or tortuous passages. Another characteristic that becomes more important with increased curvature of passages is the capacity to resist kinking, sometimes referred to as xe2x80x9ckinkability.xe2x80x9d
The quest to provide treatment options for narrower vessels and other lumens has given rise to the need to reduce catheter sizes, yet retain their favorable structural properties. One successful approach involves providing a metal reinforcement layer within a polymeric catheter body. While several of the aforementioned patents concern this approach, reference also is made to U.S. Pat. No. 5,836,926 (Peterson, et al.), disclosing an intravascular catheter that incorporates a reinforcing layer of interbraided stainless steel wires. The reinforcing layer is situated between an inside layer of polytetrafluoroethylene, and an outer layer comprising a blend of polyetherester elastomer and polybutylene terepthalate. As compared to a similarly sized catheter without the metallic reinforcement, this catheter can achieve improved pushability and torqueability, as well as improved kink resistance, while remaining sufficiently flexible.
In catheters used to deliver stents or other prostheses (e.g., grafts), a reduction in catheter diameter limits the prostheses deliverable by the device to smaller diameter prostheses, or alternatively increases the degree of radial compression applied to a prosthesis to enable its delivery. For a radially self-expanding prosthesis, the increased radial compression also increases the radial force between the prosthesis and surrounding catheter when the prosthesis is in its reduced-radius delivery state. The increase in radial force causes a proportionate increase in the frictional force that interferes with deployment of the prosthesis after its delivery to the treatment site.
To achieve further reductions in diameter of a metallic-braid-reinforced catheter, the thickness of the polymeric wall might be reduced, either with or without reducing the size of the wires or strands of the metal reinforcement. However, reducing the diameters of the strands reduces their strength, and thus diminishes one or more of the properties associated with strength: pushability, torqueability and kink resistance. Reducing the polymeric wall thickness while maintaining the strand size, however, eventually leads to unwanted exposure of the reinforcing braid.
Therefore, it is an object of the present invention to provide a catheter or other tubular medical device with a structure that, despite a reduction in diameter, maintains favorable properties related to the strength of a reinforcing strand. Another object is to provide a catheter structure that affords enhanced capacity to control desired properties.
A further object is to provide a process for fabricating a catheter that either enhances desired properties, or retains desired properties at acceptable levels despite a reduced diameter.
Yet another object is to provide a prosthesis delivery catheter capable of accommodating larger prostheses, or that requires less radial compression of a prosthesis during its delivery to a designated treatment site within a body lumen.
To achieve these and other objects, there is provided a body insertable medical device. The device includes a pliable tubular body having an inside surface, an outside surface, and a wall thickness in a radial direction. The tubular body surrounds an interior lumen. The device further includes a reinforcing structure including at least one elongate flexible structural strand having a substantially uniform non-circular transverse profile. The profile has a thickness dimension and a width dimension at least twice the thickness dimension. The structural strand is incorporated into the tubular body between the outside surface and the inside surface in a substantially surrounding relation to the interior lumen. The structural strand is oriented with the thickness dimension in the radial direction.
Preferably a plurality of the structural strands are wound helically in opposite directions and interbraided with one another, to form multiple crossings of the structural strands. The strands are spaced apart axially to define multiple pics. The axial length of the pics, as determined by the strand spacing, can be selected to influence pushability, torqueability, flexibility and kink resistance. The transverse profiles of the strands, both as to surface area and as to the ratio of width to thickness, also can be selected to influence these characteristics. For example, structural strand strength can be increased by increasing the strand width while maintaining the same thickness. Flexibility can be increased by increasing the pic axial length. Another factor influencing the desired characteristics is the braid angle of the filament strand windings, i.e., the angle of each helical strand with respect to a longitudinal central axis. Increasing the braid angle tends to increase the torqueability while reducing the pushability. In short, strands and arrangements of strands can be selected to customize catheter designs, to an extent not possible in braids formed of the conventional strands having circular profiles.
In one particularly preferred construction, the strands have rectangular transverse profiles, with width-to-thickness ratios in the range of 3.0 to 6.6.
In a specific example, a stainless steel braid features structural strands having a rectangular transverse profile in which the width is 0.003 inches, and the thickness is 0.0007 inches. This arrangement maintains the desired pushability, torqueability and kink resistance, e.g., as compared to a braid employing conventional strands having circular transverse profiles with 0.003 inch diameters. Further, satisfactory flexibility results from a pic rate of about 80-100 pics per inch, i.e., a pic axial length in the range of 0.01 inches to 0.0125 inches.
In one advantageous construction, the tubular body comprises an inside layer formed of a low-friction polymeric material, and an outside layer surrounding the inside layer and formed of a polyether ester or a polymeric amide. The structural strand reinforcement is situated between the inside layer and the outside layer. The inside layer provides lubricity, i.e., reduced friction to ease the deployment of a stent or otherwise facilitate passage of any member within the interior lumen. The outer layer can be composed of a distal section to enhance distal end flexibility, and a less flexible proximal section to enhance pushability and kink resistance.
According to another aspect of the invention there is provided a process for fabricating a catheter, including:
a. positioning a polymeric inner tube over an assembly mandrel;
b. selecting an elongate flexible structural strand having a non-circular transverse profile characterized by a thickness dimension and width dimension at least twice the thickness dimension;
c. winding the structural strand helically over a core rod, with the thickness dimension oriented radially with respect to the core rod, thus to form a tubular support structure;
d. removing the tubular support structure from the core rod, and placing the support structure over the polymeric inner tube and assembly mandrel;
e. placing a polymeric outer tube over the tubular support structure, whereby the polymeric outer tube surrounds the polymeric structure and the inner polymeric tube;
f. placing a heat shrink tube around the polymeric outer tube to form an assembly including the heat shrink tube, the polymeric outer tube, the tubular support structure and the polymeric inner tube;
g. applying heat to the assembly sufficient to cause a radial shrinkage of the heat shrink tube, thereby radially compressing the assembly;
h. heating the radially compressed assembly sufficiently to form a fusion bond along an interface of the polymeric inner and outer tubes; and
i. after forming the fusion bond, removing the heat shrink tube.
Thus, in accordance with the present invention, catheters with metal reinforcement can be formed with reduced diameters, yet retain favorable pushability, torqueability and resistance to kinking. These characteristics, along with flexibility, can be more precisely tailored to meet varying needs. Catheters used to deploy stents and other prostheses can be fashioned to handle prostheses of larger sizes, or to require a less severe radial contraction of a radially self-expanding prosthesis to achieve its delivery state. Alternatively, a catheter can be fashioned with the same prosthesis delivery capacity as a conventional catheter, but with a reduced diameter.